Cyclic adsorption processes are generally practiced in batteries of adsorption vessels comprised of two or more adsorbent-filled vessels arranged in parallel and operated out of phase such that at least one vessel is in the adsorption mode while at least one other vessel is in the adsorbent regeneration mode.
In each cycle of the process a series of sequential steps, including adsorption, equalization and regeneration, are carried out in each vessel. To enable the various streams to flow to and from the vessels, the feed, product, and exhaust lines have been provided with a rotary valve assembly that provides valving action to permit gas flow through these lines at the appropriate time in the adsorption cycle.
The rotary valve assembly also permits communication between the inlet ends of the vessels and the outlet ends of the vessels to permit flow between the vessels during pressure equalization steps. Pressure equalization is the passage of gas from a first vessel that has just completed its adsorption step to a vented or evacuated vessel which has just completed its adsorbent regeneration step.
Relevant background art for pressure swing adsorption systems can be found in the following U.S. patents, all of which are hereby incorporated by reference for all they disclose and describe: U.S. Pat. Nos. 5,814,131, 5,814,130, 5,807,423, 5,366,541, 5,268,021, and Re. 35,009.
U.S. Pat. Nos. 5,814,130, 5,814,131 and 5,807,423 disclose a rotary valve assembly for use with adsorption vessels that generally includes a valve port disk and rotary valve. The valve port disk and the rotary valve are described as being ground to have highly polished flat finishes to enable the faces of the disks to form a fluid-tight seal with each other. The rotary valve is rotated relative to the stationary valve port disk so that openings on the face of the rotary valve register with holes in the valve port disk, providing valving action to permit appropriate gas flow through the vessels for the adsorption, regeneration and equalization modes.
Between the rotary valve and the inside surface of a valve assembly cover are a number of annular channels formed by multiple annular seal rings disposed around the valve. A respective exhaust line, purge fluid supply line and product gas line communicate with these annular channels. The rotary valve includes bores extending from the openings on the flat engagement surface to the periphery of the valve for communicating the openings with the annular channels and fluid lines.
There are a number of drawbacks with this proposed design in these patents. First, the multiple annular seal rings are impractical. The seal rings would be expensive to make, difficult to install and service, difficult to make leak-free (even when new), and would be subject to wear and increased leakage over time. The leakage between these various fluid streams could have serious negative effects on the performance of the gas separation device, i.e., the product gas would become contaminated. The seal rings also would greatly increase the torque required to turn the valve and, hence, increase the size of the motor.
Second, the rotary valve assembly is not pressure balanced. At the operating pressures needed for the separation cycle, a very heavy pre-load would need to be placed on the valve parts to prevent their separation while operating.
Third, the co-location of the purified product gas and feed gas on the faces of the rotary valve and the valve port disk would inevitably lead to leakage of the feed gas into the product gas. Feed gas is at higher pressure than the product gas and, hence, has the multiple driving forces of differential pressure and a large concentration gradient leading to contamination of the high purity product with contaminates from the feed gas. Even though the leak rate can be made very low by producing a valve face interface with sufficient accuracy, i.e., flatness and finish, the leakage can not be eliminated altogether since the valve depends on a thin gas film being established between the flat engagement surfaces of the rotary valve and the valve port disk. In the case of nitrogen separation from air, if the desired product purity is in the range of tenths of percentage points oxygen to PPM (Parts Per Million) levels of oxygen, the rotary valve assembly described in these patents could not be used.